There are several versions of omnidirectional outdoor sirens which consist of multiple stacked circular speaker arrays, or cells. These designs were generally developed decades ago. Each type of design has certain advantages, yet also certain disadvantages, such that a new improved siren can be developed as provided herein that incorporates some of the advantages, eliminates the disadvantages, and introduces new enhancements to performance, manufacturability, and serviceability.
Omnidirectional modular sirens have been available through companies such companies as Whelen Engineering, Federal Signal, and American Signal. The existing designs from each of these companies generally consist of one or more disc-shaped modules that stack vertically, within which there is one or more audio compression drivers which output sound energy directed through an expanding channel that then exits through either its lower and/or upper surface where the sound energy then spreads out omnidirectionally in the horn-shaped space between the modules.
Whelen's U.S. Pat. No. 4,908,601 describes a single, very powerful and therefore heavy and expensive 400 watt driver mounted vertically in each module. Due to the high percentage of total siren output provided by each driver, the failure of one driver can result in a measurable and noticeable degradation of siren output. Because the weight of the driver is over 10 pounds and is centrally located on a 30″ diameter structure, it is also very difficult to service.
The Whelen design is in the radial support structure of the stacking methodology. A series of 6 vertical support posts secured by nuts and bolts are positioned around the peripheral edge of each module, creating the supporting structure and strength of the stack of multiple modules. Six vertically oriented nuts and bolts are used to secure the posts of adjacent modules. Being vertically positioned, the bolts are difficult to access and require using two tools (bolt and nut end) at the same time to remove or install. The support posts are also positioned in the exiting acoustic surface, presenting multiple discontinuities in the acoustic wavefront path in all directions
Another disadvantage of the Whelen design is regarding the total length and shape of the acoustic channel (horn shape) that directs the sound energy. The Whelen design provides a horn channel that is only approximately 26″ long measured from the throat of the channel where sound enters from the driver, to the mouth of the exiting surface between the modules, which is considerably shorter than other siren designs. In addition, the parabolic and radically flared shape of the Whelen design in the final exiting surface represents a significant change (discontinuity) in the rate of expansion of the cross-sectional area of the sound energy wavefront as it transitions from the vertical tubes to the omnidirectional area between cells.
Federal Signal's U.S. Pat. No. 5,146,508 utilizes a single central mast, to which one or more stackable modules can be attached. Each module contains 4 independent acoustic waveguides (horn manifolds) that each use a single 100 watt compression driver. The sound energy travels from each driver, through its respective manifold, and is then directed downward to an opening in the module near the center mast. The sound energy enters the space between the modules and then reflects off of a conical shape near the center to radiate outward omnidirectionally.
A third omnidirectional siren design is presented is American Signal's U.S. Pat. No. 5,321,388. This patent presents a siren design which combines multiple drivers into a manifold that then splits the sound energy such that it travels into separate upper and lower exiting compartmental sections that are vertically adjacent and thereby diluting rather than concentrating or nullifying the sound. The sections are also physically limited by vertical spacers to a specific quadrant of the targeted local omnidirectional area. This design is more appropriate for applications that may require only one, two, or three quadrants of sound. In addition, the vertically-split sound manifold results in a mechanical structure that is more difficult to assemble and to service in the field when compared to designs that maintain single-level audio modules that are not linked vertically.